A Universal Mobile Telecommunications System (UMTS) or third generation (3G) network can be separated into a number of major components, namely one or more core networks which are responsible for setting up and controlling user sessions, and a UMTS radio access network (UTRAN) which controls access to the air interface. The interface between UTRAN and user equipment (UE) is provided by nodes that may be referred to as “Node B” (analogous to base stations in 2G/GSM networks) or base stations. NodeBs are responsible for transmitting and receiving data over the air interface and are controlled by radio network controllers. User and control data are routed between a base station and a core network via the base station and the radio network controllers. The interface between a base station and a radio network controller is referred to as the lub interface. The interface between two radio network controllers in the same network is referred to as the lur interface. A lu interface carries user traffic (such as voice or data) as well as control information, and is mainly needed for soft handovers. Soft handover refers to a feature used by the Code Division Multiple Access (CDMA) and Wideband Code Division Multiple Access (WCDMA) standards, where a user equipment, such as a cell phone, is simultaneously connected to two or more cells (or cell sectors) during a call. On the uplink (user equipment-to-cell-site), all the cell site sectors that are actively supporting a call in soft handover send the bit stream that they receive back to the radio network controller, along with information about the quality of the received bits. The radio network controller examines the quality of all these bit streams and dynamically chooses the bit stream with the highest quality. Again, if the signal degrades rapidly, the chance is still good that a strong signal will be available at one of the other cell sectors that are supporting the call in soft handover.
In UTRAN, the high-speed downlink shared channel (HS-DSCH) does not use soft handover as dedicated channels do. Instead, a procedure called HS-DSCH serving cell change (HSCC) is utilized to make a hard handover. Soft handover is still used for the uplink, and an active set is managed in the same way as for non-high speed user equipment. The active set comprises a list of all active cells the user equipment is connected to in uplink soft handover. The user equipment continuously measures a common pilot channel (CPICH) and comprises a hysteresis to be fulfilled during a certain time (time to trigger). The update procedure for the list of active set of cells is schematically illustrated in FIG. 1. The active set update procedure is triggered by measurement report 1a, 1b or 1c, 101 informing the network controller, e.g. a serving radio network controller SRNC, that new cells have fulfilled the criterions to be added (measurement report 1a), deleted (measurement report 1b) or replaced (measurement report 1c) in the active set. The measurement report 1a, 1b, 1c is sent from the user equipment to a network controller, e.g. a serving radio network controller SRNC. A serving radio network controller is a type of radio network controller serving a particular user equipment and manages the connections towards that user equipment. When in HS-DSCH operation, the downlink is not in soft handover. Instead, one of the cells (typically the strongest) in the active set is marked as current HS-DSCH serving cell. The network controller then performs a radio link addition 102, and sets up the required radio links by sending and receiving setup request and response 103, 104 to/from the base station. The network controller transmits an active set update message 105 to the user equipment. When the user equipment has received the active set update from the network controller, it prepares 106 a processing, i.e. it reads the message and applies the new configuration, e.g. adds or deletes a radio link. The user equipment sends an active set update complete message 107 to the network controller confirming that the active set update was complete. The duration of an active set update procedure may, as an example, be calculated as follows:Tasu=Ttrig1a+2*TUu+2*Tlub                 where                    Ttrig1a=Time to trigger measurement report 1a=320 ms            TUu=Uu (Radio Interface) delay=100 ms            Tlub=lub (Radio Network Controller−NodeB Interface) delay=10 ms            Tasu=Time for active set update                        
The signalling sequence for a (regular) HSCC procedure for hard handover is schematically shown in a combined flow and signalling diagram in FIG. 2. The user equipment performs a handover evaluation 200 to determine whether a handover shall be performed. This is triggered by a neighbour cell, target cell, being stronger than of the current cell, serving cell. A measurement report 1d is sent 201 from the user equipment to the network controller, e.g. the serving radio network controller (SRNC), indicating that another cell in the list of active set of cells has become the strongest one. As an example, the measurement report 1d is triggered to be sent from the user equipment when the measured common pilot channel level (CPICH) of the target cell is stronger than the serving cell by a certain hysteresis for a given time, governed by a parameter Ttrig1d (time to trigger measurement report 1d).
When not considering possible processing delays in the network controller and the user equipment, i.e. in simulation, the delay of the cell change procedure, Tcc, used may, as an example, be calculated as follows:Tcc=Ttrig1d+TUu+2*Tlub+TActivationTime                 where                    Tcc=Delay of cell change procedure            Ttrig1d=time to trigger measurement report 1d             TUu=Uu (Radio Interface) delay=100 ms            Tlub=lub (Radio Network Controller−NodeB Interface) delay=10 ms            TActivationTime=Activation time                        
When the network controller receives the measurement report 1d indicating the existence of this stronger cell, the network can take the decision to change the serving cell, i.e. it takes a handover decision 202. When a cell change is triggered, the network controller configures the source and target base stations (shown as only one base station in FIG. 2) with the new configuration, and the network controller also configures the lub transport bearer. The network controller sends a radio link reconfiguration prepare message 203 to the base stations, and receives in return a radio link reconfiguration ready message 204 when the reconfiguration is ready. When both base stations (serving and target) have acknowledged the configuration, the network controller calculates the activation time 205 for the new configuration in case the switch to the new configuration is a synchronized procedure, meaning that the user equipment and the network controller shall move to the new configuration at the same time. The calculated activation time is relative to a connection frame number (CFN). An offset is needed to cover for the time it takes to transmit the reconfiguration commit messages 206 to both the user equipment and the base stations. The network controller sends a physical channel reconfiguration message 207 to the user equipment. The user equipment prepares a processing 208, i.e. it reads the message from the network controller and executes the handover 209, i.e. applies the new configuration, e.g. adds or deletes a radio link for the handover. When the handover is complete, the user equipment may send a physical channel reconfiguration complete message 210 to the network controller.
There is however a problem for user equipments travelling at very high speed, since the link quality of the serving cell, i.e. source cell, may degrade before the cell change procedure to the target cell is completed. If this happens before the network controller is able to successfully transmit the physical channel reconfiguration message 207, the network controller will no longer be able to reach the user equipment and the call will be dropped.
Enhancements to the HS-DSCH serving cell change procedure are consequently required regarding radio protocol procedures and structures, lub/lur protocols and user equipment, base station and radio resource management (RRM) performance requirements.